After Maigrot and Sabate introduced an ion exchange membrane in 1890 while removing inorganic ions from sugar syrup using permanganic acid paper as a separation membrane, the ion exchange membrane has been widely used in various industrial fields. A process of separating and purifying a material using the ion exchange membrane is simple and has excellent selectivity for a specific ion, such that this process may be widely applied. An ion exchange membrane capable of selectively separating cations and anions in an aqueous solution has been widely used in a fuel cell, a secondary battery, a flow battery, a water-splitting electro-dialysis for recovering acid and base, diffusion dialysis for recovering acid and metal chemical species from pickling waste acid, an ultra pure water process, and the like.
Particularly, as uses of electronic products such as a small sized notebook, a mobile phone, and the like, have rapidly increased, recently, a demand for the development of the fuel cell using the ion exchange membrane has increased.
In the fuel cell, chemical energy is changed into electric energy by an electrochemical reaction Among the fuel cells, a polymer electrolyte fuel cell using a cation exchange membrane has excellent output characteristics, a low operation temperature, fast startup and response characteristics as compared to other fuel cells, such that the polymer electrolyte fuel cell may be variously applied in a distributed power supply for a house or a public building, a small power supply for electronic devices, or the like, as well as a portable type power supply for a vehicle. This polymer electrolyte fuel cell has been widely studied as an electrochemical apparatus for a convenient and efficient power resource.
A polymer electrolyte used as a cation exchange resin or cation exchange membrane in the fuel cell has been used for a long period of time and steadily studied. A cation exchange membrane, which is a cation exchange membrane, provides a layer for separately maintaining fuel and an oxidant in addition to having low resistance against diffusion of a proton from one electrode to the other electrode. In order to obtain high efficiency of the fuel cell, the cation exchange membrane should have high ionic conductivity and chemical, thermal, mechanical, and electrochemical stability. In addition, since the ionic conductivity is rapidly decreased at the time of dehydration, the cation exchange membrane should have resistance against dehydration.
Further, the cation exchange membrane should have durability and thermal, physical, and chemical stability so that the fuel cell may operate at a high temperature of 90 or more and in an ultimately acidic environment. Therefore, in addition to research into the cation exchange membrane, various researches into a cation exchange membrane used in a direct methanol fuel cell (DMFC), a polymer electrolyte membrane fuel cell (PEMFC, solid polymer electrolyte fuel cell, solid polymer fuel cell, or cation exchange membrane fuel cell) as a carrier transporting cations have been recently conducted.
Currently, as the cation exchange membrane widely and commonly used in a fuel cell field, there is Nafion® (Dupont, USA), which is a perfluoro sulfonic acid based cation exchange membrane. This fluorine based polymer has excellent mechanical properties, chemical stability, and ion conductivity but has disadvantages such as complicated synthetic materials or fabrication process, a high production cost, a low driving temperature (<100° C.), low stability in methanol, and the like.
Therefore, in order to overcome these disadvantages, a cation exchange membrane using a cheap hydrocarbon based polymer having excellent physical properties has been actively developed. A representative example of the hydrocarbon based polymer may include sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(aryl ether sulfone), a sulfonated phenol formal resin, sulfonated poly(phenylene oxide), phosphonic poly(phenylene oxide), sulfonated poly(benzylimidazole), and the like.
However, since ion conductivity of the cation exchange membrane as described above is in proportion to a sulfonation degree, in the case in which the sulfonation degree exceeds critical concentration, a decreased in a molecular weight may be inevitable. In addition, the mechanical properties of the cation exchange membrane may be decreased after being hydrated, and in this case, the membrane may not be used for a long time. In order to solve these problems, a method of preparing a polymer by using a sulfonated monomer and a method of selectively sulfonating a polymer have been developed (U.S. Pat. Nos. 5,468,574, 5,679,482, and 6,110,616), but the problem such as stability at a high temperature and problems generated at the time of use for a long period of time have not been completely solved.
A cation exchange membrane using sulfonated poly(ether ether ketone) (SPEEK) among the hydrocarbon based polymers has excellent mechanical properties and thermal stability and may be cheaply prepared. However, in the case of excessively introducing a sulfonic acid group in order to improve ion conductivity, since SPEEK excessively absorbs water, the mechanical strength and dimensional stability of the cation exchange membrane are rapidly decreased, such that there is a limitation in using the cation exchange membrane as a fuel cell membrane. In order to solve this problem, research into a technology of introducing a crosslinking structure in SPEEK to improve dimensional stability, mechanical property, and chemical stability under fuel cell driving conditions has been actively conducted.
In order to introduce a crosslinking structure in a hydrocarbon based polymer electrolyte membrane for a fuel cell, various methods such as a method of using heat or ultraviolet (UV) rays, and the like, have been used. In the case of crosslinking by means of heat, a long crosslinking process at a high temperature is required, and in the case of crosslinking by means of UV rays, it may be impossible to form a uniform crosslinking structure due to contamination caused by use of an initiator and low transmittance. Meanwhile, a crosslinking technology using radiation has advantageous in that the initiator is not required, a dense crosslinking structure may be formed up to an internal portion of a polymer electrolyte membrane due to high transmittance of the radiation, and a time consumed in a preparation process may be decreased.
The present inventor has developed a hydrocarbon based polymer electrolyte membrane having improved thermal stability and dimensional stability by mixing a crosslinker mixture having a vinyl ester structure and a hydrocarbon based polymer and irradiation (Korean Patent No. 10-1267979). However, in the hydrocarbon based polymer electrolyte membrane, in the case of using an ester group in a crosslinker structure for a long period of time, the crosslinker may be decomposed. Therefore, research into a novel material capable of having excellent thermal, mechanical, and chemical stability and increasing dimensional stability by introducing a crosslinking structure in a hydrocarbon based polymer electrolyte membrane using radiation and capable of being easily prepared has been urgently demanded.